A wireless communication system has been prevalently developed so as to provide various types of communication services, such as audio, data, or the like. A goal of the wireless communication system is to enable multiple users to execute reliable communication regardless of location and mobility. However, a wireless channel has abnormal characteristics, such as a fading phenomenon due to path loss, noise, or multipath, a Doppler effect due to intersymbol interference (ISI) or mobility of a user equipment, or the like. Therefore, various technologies to overcome the abnormal characteristics of the wireless channel and increase the reliability of the wireless communication have been developed.
As a technology for supporting reliable high-speed data services, there is multiple input multiple output (MIMO). The MIMO scheme improves the transmit and receive efficiency of data by using a multiple transmit antenna and a multiple receive antenna. An example of the MIMO scheme may include spatial multiplexing, transmit diversity, beam forming, or the like.
A MIMO channel matrix is generated by the multiple receive antenna and the multiple transmit antenna. A rank may be obtained from the MIMO channel matrix. The rank is the number of spatial layers. The rank may be defined as the number of spatial streams that may be simultaneously transmitted by a transmitter. The rank is referred to as a spatial multiplexing rate. When the number of transmit antennas is Nt and the number of receive antennas is Nr, the rank R becomes R≦min {Nt, Nr}.
In the wireless communication system, signals known to both of the transmitter and the receiver are needed for channel measurement, information demodulation, or the like. The signals known to both of the transmitter and the receiver are referred to as a reference signal (RS). The RS may be referred to as a pilot.
The receiver may estimate a channel between the transmitter and the receiver using the RS and demodulate information using the estimated channel. When the user equipment receives the RS from a base station, the user equipment may measure a channel using the RS and feedback channel state information to the base station. Since the signals transmitted from the transmitter use the corresponding channels for each transmit antenna or each spatial layer, the RS may be transmitted for each transmit antenna or each spatial layer.
Meanwhile, in the International Telecommunication Union (ITU), an international mobile telecommunication-advanced (IMT-A) system has been standardized as a next generation mobile communication system after the third generation. A goal of the IMT-A system is to provide a high transmission rate that is a 1 gigabits per second (Gbps) downlink and 500 megabits per second (Mbps) uplink while supporting internet protocol (IP) based multimedia seamless services. In the 3rd generation partnership project (3GPP), a long term evolution-advanced (LTE-A_) system is considered as a candidate technology for the IMT-A system.
Although the LTE system is required to support up to four transmit antennas for downlink transmission, the LTE-A system is required to support up to eight transmit antennas for the time of the downlink transmission. However, the user equipment (hereinafter, LTE user equipment) to which the LTE system is only applied and the user equipment (hereinafter, LTE-A user equipment) to which the LTE-A system is applied may coexist in a cell. The LTE-A system is designed to support the LTE user equipment and the LTE-A user equipment together. However, when supporting the transmission through different numbers of antennas, a problem in an apparatus and method for transmitting RS is caused. Therefore, There is a need for an apparatus and method for efficiently transmitting RS in the wireless communication system.